The commercial production of certain metals, particularly aluminum and magnesium, has been typically accomplished by the Hall-Heroult Process. In this well known process, a purified source of the metal is dissolved in a molten salt bath particularly consisting of cryolite. All chloride salts, fluoride salts or heavy salts such as barium are then electrolytically recovered at a cathode. A characteristic advantage of this type of process is low cell power efficiency due to the low electrical conductivity of the electrolytic bath, since at least a portion of the electrical energy applied to the cell is converted by resistance to heat. The advantage of the Hall-Heroult Process (that is, cell power efficiency) has reached a practical limit in energy-saving efficiency despite careful design and operation of 150 to 200K. amp. cells at anode current densities ranging between 4.5 and 5.5 amp. per square inch. The lowest energy consumption limit appears to be about 5.6 to 6.0 kilowatt hours per pound of aluminum recovered utilizing the most advanced designs, computer controls, staff modifications, and other improvements.
One approach to solving this particular problem is illustrated by U.S. Pat. No. 4,338,177 issued to Withers et al. for an electrolytic cell for the production of aluminum. In this patent, there is disclosed an electrolytic cell comprising an anode of aluminum oxide and a reducing agent, a cathode, and a molten electrolyte which does not dissolve the aluminum oxide compound in substantial quantities when the temperature of the electrolyte is in the range of 650.degree. to 900.degree. C. The cell includes a porous membrane which separates the anode and the cathode, and comprises a material having a connected pore size sufficiently small to screen out the mixture of aluminum oxide and reducing agent, but sufficiently large to pass the aluminum ions therethrough. The energy consumption for such a cell ranges from about 3.67 to 4.25 kwh/lb. of aluminum recovered. The anode and cathode spacing of Withers et al. ranges from about 0.25 to about 1.0 inch, although the smaller spacing has been shown by Withers et al. to result in less efficiency due to the liberation of carbon dioxide from the anode reaction and subsequent back reaction with the aluminum in the cathode layer. Withers et al. redesigned the cell to eliminate this problem and efficiencies were measured at about 92 percent. The electrolytic fluoride or chloride bath in Withers et al. typically contained a halide of aluminum, either chloride or fluoride, in percentages which ranged from about 1 to 95 percent. In the chloride cycle, Withers et al. required some portion of the aluminum oxide in the bath, presumably to avoid the anode effect.
In a treatise entitled "Principles of Magnesium Technology," the miminum energy required for recovery of aluminum and magnesium was disclosed as 3.5 and 3.0 kwh/lb. of metal recovered, respectively. Magnesia was noted as being insoluble in chloride salt baths. Lithium chloride was shown to have high conductivity when compared to typical bath materials, but due to its high cost was not recommended for use unless there was a sufficient improvement in cell design to justify such cost. The treatise also disclosed a Dow Lithium Chloride process in which the electrolyte comprised between 5 and 38 weight percent magnesium chloride; the balance, apart from the small addition of alkali or alkaline earth fluorides, being lithium chloride. In this recovery process for magnesium, the magnesium chloride content was held above 5 to prevent liberation of lithium with the recovered magnesium. The magnesium chloride content was limited to 38 percent to maintain a desirable density difference between the metals and the malts. Energy efficiencies ranged from 4.5 to 5.0 kwh/lb. of magnesium recovered and a narrow electrode gap of 1 was utilized.